Class-D type amplifiers are essentially switching devices where a load is connected to a power supply by low impedance switches. The low impedance switches switch between active and inactive states at frequencies much higher than a bandwidth of an input signal, and the load filters the output signal of the amplifier such that only the low frequency components of the output signal can be subsequently used. This filtering of the high frequency components of the output signal is either done explicitly in the form of LC (inductor-capacitor) circuits or implicitly as in the case of an audio speaker where the speaker is inherently a device which functions as a low-pass filter. Class-D type amplifiers are popular because they are capable of delivering a large amount of power to a load without dissipating power in the amplifier itself, thus resulting in a very high efficiency of transferring power from a power supply to a load.
FIG. 1(a) illustrates a prior art H-bridge circuit 100, the most common switch arrangement for class-D type amplifiers. The H-bridge circuit 100 includes a plurality of switches (101, 102, 103, 104) which couple a load 105 between a power supply PWR and ground GND. During a first state of operation, the switches (101, 102, 103, 104) are controlled such that a first terminal 120 of the load 105 is connected to the power supply PWR while a second terminal 121 of the load 105 is connected to ground GND. During a second state of operation, the switches (101, 102, 103, 104) are controlled such that the first terminal 120 of the load 105 is connected to ground GND while the second terminal 121 of the load 105 is connected to the power supply PWR. By switching between the first and second states of operation the connection of each side of the load to either the power supply or the ground can be periodically switched, resulting in a signal in the shape of a square wave being applied across the load. This is referred to as binary modulation since the voltage across the load has only two states.
In high performance applications, an LC filter is used to block the high frequency energy of the signal which is output from the H-bridge circuit 100, thus allowing only the low frequency energy to be applied to the load 105. The high quality inductors and capacitors required for this filter are relatively large and expensive, so cost-sensitive applications sometimes opt for filterless operation. In filterless operations, there is no LC filter used to block the high frequency energy of the signal which is output from the H-bridge circuit 100. Consequently, all of the high frequency energy of the signal which is output from the H-bridge circuit 100 is applied to the load 105, resulting in more power dissipation in the load 105 than the case where an LC filter is used.
The switches (101, 102, 103, 104) may also be controlled in a first variation of a third state of operation such that both of the first terminal 120 of the load 105 and the second terminal 121 of the load 105 are connected to the power supply PWR. Alternatively, the switches (101, 102, 103, 104) may be controlled in a second variation of the third state of operation such that both of the second terminal 120 of the load 105 and the second terminal 121 of the load 105 are connected to ground GND. By operating the H-bridge circuit 100 in three states of operation, the excess power dissipation in the load 105 for filterless operation is significantly reduced. Operating an H-bridge circuit 100 in three states of operation is referred to as ternary modulation.
Provision for a third state of operation, while having the desired effect on reducing the excess power dissipation in the load 105 for filterless operation, introduces a new problem. For an H-bridge circuit 100 which is operated in two states, the voltage applied to the load 105 is always purely differential. Consequently, the common-mode voltage of the load 105, defined as the average of the voltage at each of the first terminal 120 of the load 105 and the second terminal 121 of the load 105, or equivalently the current through the load 105, never changes. The common-mode voltage is always approximately equal to the voltage of the power supply divided by two. When provision is made for the H-bridge circuit 100 to operate in a third state, for example the aforementioned second variation where both of the second terminal 120 of the load 105 and the second terminal 121 of the load 105 are connected to ground GND, the common-mode voltage of the load 105 is equal to the ground voltage. Similarly, no current will flow through the load 105. Thus, when an H-bridge circuit 100 is operated in three states of operation, the common-mode voltage and current of the load 105 will vary based on the state of operation.
If every element of the H-bridge circuit 100 and load 105, including wiring parasitics, is perfectly balanced, the variation of the common-mode voltage and current of the load 105 is not a problem, at least from a signal integrity standpoint. However, the inevitable mismatches between the two sides of the H-bridge circuit 100 and the load 105 will cause voltages applied to the two terminals of the load 105 to not be identical. Some of this variation in common-mode voltage will show up as a differential voltage and a current through the load 105, consequently corrupting the output signal.
In order to solve the problem of variations in the common-mode voltage and current of the load 105, it is desired to modulate the third state of operation between each of the variations of the third state of operation. By switching between the two variations for the third state of operation, variations in the average common-mode voltage and current will be reduced.
FIG. 1(b) illustrates a prior art device that attempts to address this problem as described by M. Corsi et. Al. in U.S. Pat. No. 6,262,632 B1. The circuit implementation described by M. Corsi includes two pulse width modulators (150, 160) which each include a single-ended differential amplifier and a comparator for generating a pulse-width modulated signal from an audio input signal that is subsequently applied to half of an H-bridge circuit 100. One of the pulse width modulators 150 and its corresponding half H-bridge circuit construct a positive half circuit, while the other one of the pulse width modulators 160 and its corresponding half H-bridge circuit construct a negative half circuit. The positive and negative half circuits operate in inverse phase with respect to one another and provide signals (outp, outn) which are subsequently applied to a load 105.
The method for controlling the H-bridge circuit 100 illustrated in FIG. 1(b) has many deficiencies. For example, this method only works for pulse width modulators. Further, this method requires multiple single-ended differential amplifiers, consequently resulting in a large spatial implementation and the many disadvantages resulting from such a large spatial implementation.
The present invention is provided to resolve the above-described problems. The objectives of the present invention are to provide an apparatus and method that reduce variations in an average common-mode voltage and current applied to a load via an H-bridge circuit while reducing the spatial requirements of the implementation of such an apparatus and method.